Troubleshooting Layer 3 Problems
This chapter suggests that the best starting place to troubleshoot serial links is to ping the IP address of the router on the other end of the link—specifically, the IP address on the serial link. Interestingly, the serial link can be in an up and up state but the ping can still fail because of Layer 3 misconfiguration. In some cases, the ping may work but the routing protocols might not be able to exchange routes. This short section examines the symptoms, which differ slightly depending on whether HDLC or PPP is used and the root cause.
First, consider an HDLC link on which the physical and data link details are working fine. In this case, both routers’ interfaces are in an up and up state. However, if the IP ...
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